12 NERVOUS SYSTEM AND SENSE-ORGANS 
advantageous in regard to correlation of the different parts of the 
body, and adaptation to the environment. It may also be noted 
that while in many lower Arthropods the two halves of the ventral 
cord are more or less separate, they are intimately united together 
in higher forms. 
Nervous Systems of Crustaceans (Crustacea). — Successive 
stages of fusion in the nervous system may be illustrated by 
comparison of Apus, Crayfish, and Crab. In the first of these, 
which is one of the lower forms, there is a 
nerve-ring with clearly-marked brain, and 
a ventral cord of which the two halves are 
widely separate (fig. 1014). The brain of a 
typical Annelid, such as the Sea-Centipede 
(Nereis), is lodged in a head-lobe (prosto- 
mium) that forms the front of the head and 
overhangs the mouth, and it supplies with 
nerves the eyes and feelers which are borne 
upon this lobe (fig. 1014). The brain of 
Apus is placed in a corresponding position, 
and is in the main equivalent to that of 
Nereis, though probably not entirely so. A 
Fig. 1014.—Front Part of the Crustacean possesses two pairs of feelers 
Central Nervous System of AP, (antennules and antennz) situated in front 
6, Fics wice guts ame Of the mouth, but most likely their original 
through nerve-ring: Gy1-G4, position was behind that aperture, and they 
ganglia of one half of ventral cord; . : 
Opn, Ants, Antz, Md, Mx1, have shifted forwards into a position more 
Mzx.2, Th/.1, nerves to eyes, anten- 
nules, antennz, mandibles, firstand Suitable for the work they have to perform, 
second maxillz, and first thoracic a - 
feet; V., visceral nervous system. Dy Way of exploring the surroundings. 
These two pairs of feelers belong to two 
segments of the head, each of which is provided with a cor- 
responding pair of ganglia. In Apus those of the segment to 
which the antenne belong are the first pair of the ventral cord 
(see figure), but the nerve for each front feeler or antennule arises 
from the side of the nerve-ring, and can be traced into the brain. 
This is intelligible if we suppose that organ to be equivalent to the 
brain of an Annelid, plus the ganglia supplying the antennules, 
which have shifted forwards along the sides of the nerve-ring. If 
this view be correct, a certain amount of fusion and centralization 
has taken place at the front end of the nervous system in Apus, as 
compared with an Annelid. But it is here necessary to state that 
